{"gene":"IGFBP4","run_date":"2026-06-10T01:55:22","timeline":{"discoveries":[{"year":1995,"finding":"IGFBP-4 inhibits IGF action by preventing binding of IGF-I and IGF-II to the type I IGF receptor; IGFBP-4 decreased binding of both 125I-IGF-I and 125I-IGF-II to bone cells and decreased binding of IGF-I tracer to purified type I IGF receptor. IGF analogs with >100-fold reduced affinity for IGFBP-4 were not inhibited, confirming the mechanism requires ligand sequestration.","method":"Radioligand binding assay with purified type I IGF receptor; cell proliferation assays with IGF analogs having reduced IGFBP-4 affinity","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — multiple orthogonal methods (receptor binding, analog competition, cell proliferation), replicated across multiple binding protein family members in same study","pmids":["7544787"],"is_preprint":false},{"year":1995,"finding":"Proteolytic cleavage of IGFBP-4 occurs between residues Lys-120 and Met-121 (the non-homologous midregion); the resulting ~16 kDa N-terminal fragment no longer inhibits IGF-1-stimulated thymidine uptake. Mutagenesis of Lys-120 to Ala (K120A) produced a protease-resistant IGFBP-4, confirming this as the cleavage site.","method":"Electrospray mass spectrometry, N-terminal amino acid sequencing, site-directed mutagenesis (K120A), thymidine incorporation bioassay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — reconstitution with purified proteins, mass spectrometry, and confirmatory mutagenesis in single rigorous study","pmids":["7538115"],"is_preprint":false},{"year":2001,"finding":"IGF-II enhances IGFBP-4 proteolysis by PAPP-A by binding to IGFBP-4 (not to PAPP-A); pre-incubation of IGFBP-4 with IGF-II followed by removal of free IGF-II was sufficient for proteolysis, whereas pre-incubation of PAPP-A with IGF-II was not. IGFBP-4 mutants lacking IGF-binding activity (deletions of Leu72-His74 or Cys183-Glu237) were resistant to cleavage even in the presence of IGF-II.","method":"Cell-free protease assay with purified PAPP-A; pre-incubation/removal experiments; IGFBP-4 deletion mutants lacking IGF-binding activity","journal":"Archives of biochemistry and biophysics","confidence":"High","confidence_rationale":"Tier 1 / Strong — mechanistic dissection with purified proteins, multiple loss-of-function mutants, and pre-incubation controls in one study","pmids":["10898936"],"is_preprint":false},{"year":2001,"finding":"PAPP-A is the predominant IGFBP-4 protease in human pregnancy serum; immunoprecipitation of PAPP-A completely abolished both IGF-II-dependent and IGF-II-independent IGFBP-4 proteolytic activity in pregnancy serum. PAPP-A also enhances IGF bioactivity in vitro by degrading IGFBP-4, as shown using protease-resistant IGFBP-4.","method":"Immunoprecipitation/immunodepletion of PAPP-A from pregnancy serum; cell proliferation assay with wild-type vs. protease-resistant IGFBP-4; PAPP-A neutralizing antibody","journal":"The Journal of clinical endocrinology and metabolism","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal immunodepletion and neutralizing antibody confirm identity of protease; functional rescue with protease-resistant mutant","pmids":["11158056"],"is_preprint":false},{"year":2001,"finding":"PAPP-A cleaves IGFBP-5 independent of IGF, but cleavage of IGFBP-4 by PAPP-A can also occur in the absence of IGF at a very slow rate; IGF binds to IGFBP-4 to make it a better substrate for PAPP-A rather than binding directly to PAPP-A.","method":"In vitro cleavage assay using highly purified recombinant proteins; rate comparisons with and without IGF; IGF binding competition with PAPP-A","journal":"FEBS letters","confidence":"High","confidence_rationale":"Tier 1 / Strong — highly purified recombinant proteins, multiple substrate comparisons, direct mechanistic conclusion supported by negative binding result (IGF does not bind PAPP-A)","pmids":["11522292"],"is_preprint":false},{"year":1999,"finding":"The IGF-II-dependent IGFBP-4 protease in human osteoblasts cleaves IGFBP-4 at Met135-Lys136 as the sole cleavage site; residues 94-119 (containing no cleavage site) are required for protease susceptibility, suggesting they maintain the IGFBP-4 conformation needed to expose the cleavage site. Protease-resistant IGFBP-4 analogs were more potent inhibitors of IGF-II-induced cell proliferation in protease-producing osteoblasts than wild-type.","method":"N-terminal amino acid sequencing and mass spectrometry of proteolytic fragments of His-tagged IGFBP-4; deletion mutants; cell proliferation assay comparing wild-type vs. protease-resistant analogs","journal":"Journal of bone and mineral research","confidence":"High","confidence_rationale":"Tier 1 / Strong — mass spectrometry and sequencing of cleavage products, multiple deletion mutants, functional validation in osteoblasts","pmids":["10620067"],"is_preprint":false},{"year":2008,"finding":"IGFBP-4 functions as a Wnt signaling inhibitor independent of IGF binding: IGFBP-4 physically interacts with Frizzled 8 (Frz8) and LRP6, blocking Wnt3A binding to these receptors and inhibiting canonical Wnt/β-catenin signaling. This cardiogenic activity was IGF-independent but was attenuated by IGFs through IGFBP-4 sequestration.","method":"Co-immunoprecipitation (IGFBP-4 with Frz8 and LRP6); competitive binding assay (Wnt3A displacement); in vitro cardiomyocyte differentiation; Igfbp4 knockdown in vitro and in vivo","journal":"Nature","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal Co-IP of IGFBP-4 with two Wnt receptor components, competitive binding data, and loss-of-function in both cell culture and in vivo","pmids":["18528331"],"is_preprint":false},{"year":2016,"finding":"IGFBP-4 protects the ischemic heart by inhibiting β-catenin signaling, without inducing LRP5/6 endocytosis/degradation (unlike Dkk1). Direct intracardiac injection of recombinant IGFBP-4 post-MI inhibited β-catenin while preserving LRP5/6 protein levels, defining a mechanistic distinction between IGFBP-4 and Dkk1 as Wnt inhibitors.","method":"Conditional cardiomyocyte-specific LRP5/6 and β-catenin knockout mice; direct intracardiac injection of recombinant IGFBP-4 and Dkk1; surgical MI model; Western blot for β-catenin","journal":"Circulation","confidence":"High","confidence_rationale":"Tier 2 / Strong — in vivo KO mice with defined phenotype, recombinant protein injection, mechanistic comparison of two inhibitors with distinct downstream effects","pmids":["27803037"],"is_preprint":false},{"year":2002,"finding":"A protease-resistant IGFBP-4 mutant (IGFBP-4.7A, with 7 basic residues in the cleavage domain substituted to alanines) is stabilized in vivo in smooth muscle tissue of transgenic mice and produces greater smooth muscle hypotrophy than equivalent expression of native IGFBP-4, demonstrating that proteolytic processing of IGFBP-4 reduces its inhibitory activity in vivo.","method":"Targeted expression of protease-resistant vs. native IGFBP-4 in transgenic mice (smooth muscle alpha-actin promoter); tissue weight measurements; Western blot quantification of protein levels","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Strong — in vivo transgenic comparison of native vs. protease-resistant mutant with quantitative protein and phenotype measurements","pmids":["11923290"],"is_preprint":false},{"year":2001,"finding":"Systemic administration of wild-type IGFBP-4 (but not protease-resistant IGFBP-4) increases bone formation parameters and free IGF-I in serum, indicating that proteolytic degradation of systemically administered IGFBP-4 releases IGF and increases IGF bioavailability in vivo.","method":"In vivo biochemical markers (osteocalcin, ALP) in mice; free IGF-I measurement; comparison of wild-type vs. protease-resistant IGFBP-4; IGF-I-deficient mouse model","journal":"Endocrinology","confidence":"High","confidence_rationale":"Tier 2 / Strong — in vivo mouse study comparing wild-type vs. protease-resistant IGFBP-4 with multiple bone formation markers and IGF-I measurement","pmids":["11356715"],"is_preprint":false},{"year":1998,"finding":"Overexpression of IGFBP-4 in smooth muscle cells of transgenic mice (via smooth muscle alpha-actin promoter) causes smooth muscle hypoplasia in bladder, aorta, intestine, uterus, and stomach, demonstrating IGFBP-4 functions as a functional in vivo antagonist of IGF-I action on smooth muscle cell growth.","method":"Transgenic mouse overexpression (SMC-specific); organ weight measurements; in situ hybridization for transgene localization; Western ligand blot for IGFBP-4 protein levels","journal":"Endocrinology","confidence":"High","confidence_rationale":"Tier 2 / Strong — six independent transgenic lines with consistent phenotype, tissue-specific expression confirmed by ISH","pmids":["9564877"],"is_preprint":false},{"year":2003,"finding":"Paracrine overexpression of IGFBP-4 in osteoblasts (via osteocalcin promoter transgene) causes a 25-fold increase in bone IGFBP-4, reduces IGFBP-5, decreases osteoblast number and bone formation rate by ~50%, and induces global postnatal growth retardation with disproportionate reduction in bone size, attributed to sequestration of IGF-1.","method":"Transgenic mouse overexpression (osteocalcin promoter); histomorphometry; Western ligand blot; organ allometry","journal":"Journal of bone and mineral research","confidence":"High","confidence_rationale":"Tier 2 / Strong — bone-specific transgenic overexpression with quantitative histomorphometry confirming cellular mechanism","pmids":["12733722"],"is_preprint":false},{"year":2004,"finding":"PAPP-A is the IGFBP-4 protease in MC3T3-E1 osteoblasts; immunodepletion of PAPP-A from MC3T3-E1 conditioned medium abolished IGFBP-4 degradation; mutation of basic residues near the PAPP-A cleavage site in IGFBP-4 inhibited degradation; PAPP-A mRNA is expressed throughout osteoblast differentiation.","method":"Immunodepletion of PAPP-A from conditioned medium; IGFBP-4 mutants with basic residue substitutions near cleavage site; RT-PCR for PAPP-A mRNA","journal":"Biochemical and biophysical research communications","confidence":"High","confidence_rationale":"Tier 2 / Strong — immunodepletion combined with site-directed mutant substrates confirms both enzyme identity and cleavage site requirements","pmids":["15341545"],"is_preprint":false},{"year":1997,"finding":"IGFBP-3, -5, and -6 inhibit IGFBP-4 proteolysis through highly basic, heparin-binding C-terminal domains (thyroglobulin type-1 motifs); synthetic peptides from these regions inhibit IGFBP-4 degradation with different potencies; IGFs reverse the inhibitory effect of these IGFBPs; heparin also reverses inhibition, suggesting the basic domain competitively sequesters the protease.","method":"In vitro protease assay with 125I-IGFBP-4 substrate in MC3T3-E1 conditioned medium; synthetic peptides; heparin competition assay","journal":"Endocrinology","confidence":"High","confidence_rationale":"Tier 1-2 / Moderate — in vitro reconstitution with defined peptides and multiple competitive inhibitors, single lab but multiple orthogonal approaches","pmids":["9165012"],"is_preprint":false},{"year":1995,"finding":"IGFBP-3 functions as an IGF-reversible inhibitor of IGFBP-4-degrading proteinase activity in MC3T3-E1 osteoblast conditioned medium; the inhibitory activity resides in mid-region and C-terminal basic/heparin-binding peptides of IGFBP-3, not the N-terminal IGF-binding domain; heparin partially reverses inhibition.","method":"In vitro protease assay with 125I-rhIGFBP-4; recombinant IGFBP-3 fragments; synthetic IGFBP-3 peptides; heparin competition","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro reconstitution with defined recombinant fragments and peptides, mapping of inhibitory domain to specific sequences","pmids":["7499205"],"is_preprint":false},{"year":1994,"finding":"TGF-β augments IGF-II-dependent IGFBP-4 proteolysis in human osteoblast-like cells and independently decreases IGFBP-4 mRNA expression; other bone-relevant hormones (PTH, GH, insulin, calcitonin, glucocorticoids, sex steroids, 1,25(OH)2D3, EGF) had no significant effect on IGFBP-4 protease activity.","method":"Cell-free IGFBP-4 protease assay using conditioned medium from treated hOB cells; Western ligand blot; Northern blot for IGFBP-4 mRNA","journal":"The Journal of clinical endocrinology and metabolism","confidence":"High","confidence_rationale":"Tier 2 / Strong — systematic screen of 26 donors with cell-free protease assay and mRNA analysis, two distinct regulatory mechanisms identified","pmids":["7527411"],"is_preprint":false},{"year":2000,"finding":"The IGF-dependent IGFBP-4 protease from human fibroblasts also cleaves recombinant rat IGFBP-4 at a KMKV site (carboxyl side of Met-131); this site is not present in other IGFBPs, explaining substrate specificity. Kallikrein can also cleave at this site but non-specifically.","method":"In vitro cleavage assay; mass spectrometric characterization of cleavage site; kallikrein comparison","journal":"Growth hormone & IGF research","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — mass spectrometry defines cleavage site, single lab study with limited functional follow-up","pmids":["11161967"],"is_preprint":false},{"year":1992,"finding":"IGFBP-4 (both glycosylated and non-glycosylated forms) crosses the capillary boundary of the isolated perfused rat heart and distributes preferentially in subendothelial connective tissue (connective tissue/cardiac muscle ratio ~20-27:1), whereas IGFBP-1, -2, -3 and IGF-I preferentially localize in cardiac muscle. The connective tissue distribution of endothelial cell IGFBPs is accounted for specifically by IGFBP-4.","method":"Perfusion of isolated beating rat hearts with purified glycosylated and non-glycosylated IGFBP-4; autoradiographic distribution analysis; comparison with other IGFBPs","journal":"Endocrinology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct localization experiment in intact organ with two IGFBP-4 forms and comparison against four other IGF-binding proteins","pmids":["1377125"],"is_preprint":false},{"year":2001,"finding":"The C-terminal basic domain of IGFBP-3 (20-amino-acid stretch) determines cardiac tissue distribution; chimeric IGFBP-3(4) (IGFBP-3 with IGFBP-4's C-terminal region) localizes in connective tissue like IGFBP-4, while IGFBP-4(3) (IGFBP-4 with IGFBP-3's C-terminal region) localizes in cardiac muscle like IGFBP-3. IGFBP-4(3) also binds microvascular endothelial cells with higher affinity than IGFBP-3.","method":"Chimeric IGFBP construction; perfused rat heart distribution studies; 125I-IGFBP binding to microvascular endothelial cells","journal":"Endocrinology","confidence":"High","confidence_rationale":"Tier 2 / Strong — domain-swap chimeras directly define the C-terminal basic region as the determinant of tissue distribution, validated in perfused organ and cell binding assay","pmids":["11517150"],"is_preprint":false},{"year":2000,"finding":"Circulating N-terminal (Asp1-Phe122, Mr 13,233 Da) and C-terminal (Lys136-Glu237, Mr 11,344 Da) fragments of IGFBP-4 are present in human plasma; the N-terminal fragment retains significant IGF binding (IGF-II Kd ~17 nM, IGF-I Kd ~5 nM), whereas the C-terminal fragment has very low IGF affinity (IGF-II Kd ~690 nM, IGF-I Kd >60 nM). Disulfide bonding pattern of C-terminal fragment: Cys153-183, Cys194-205, Cys207-228.","method":"Purification from human hemofiltrate by chromatography; mass spectrometry and N-terminal sequencing; plasmon resonance spectroscopy; ligand blot; saturation/displacement binding studies; proteolytic digestion for disulfide mapping","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — multiple biochemical methods including mass spectrometry, surface plasmon resonance, and disulfide mapping on purified endogenous fragments","pmids":["10819974"],"is_preprint":false},{"year":1997,"finding":"Surface-bound plasmin (generated by u-PA/plasminogen at the HT29-D4 cell surface) selectively cleaves IGFBP-4 (>95%) without affecting IGFBP-2 or IGFBP-6 (though soluble plasmin cleaves all three), yielding 18- and 14-kDa fragments with poor IGF-II affinity, thereby releasing ~20% of cell surface-associated IGF-II for binding to IGF-I receptors.","method":"Cell-surface plasmin generation assay; Western ligand blot; 125I-IGF-II receptor binding after IGFBP-4 proteolysis; comparison of cell-bound vs. soluble plasmin","journal":"International journal of cancer","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — cell-based reconstitution with defined protease generation, comparison of substrate specificity, and IGF-II receptor binding quantification in same study","pmids":["9311602"],"is_preprint":false},{"year":1993,"finding":"1,25-dihydroxyvitamin D3 increases IGFBP-4 mRNA up to 11-fold and protein secretion 2-3 fold in human osteoblast-like cells in vitro; consistent in vivo elevation of serum IGFBP-4 was observed in human subjects treated with high-dose oral 1,25(OH)2D3.","method":"Northern blot; Western ligand blot; human subject treatment with oral 1,25(OH)2D3 for psoriasis","journal":"The Journal of clinical endocrinology and metabolism","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vitro mRNA and protein data corroborated by in vivo human treatment study in same paper","pmids":["7521341"],"is_preprint":false},{"year":2013,"finding":"SOX9 directly regulates IGFBP-4 transcription in intestinal epithelial cells; SOX9 binds the IGFBP-4 promoter as shown by ChIP; overexpression of SOX9 reduces cell proliferation which is restored by IGFBP-4 neutralizing antibody, placing IGFBP-4 downstream of SOX9 in a proliferation-suppressing pathway.","method":"Chromatin immunoprecipitation (ChIP) of SOX9 at IGFBP-4 promoter; reporter assay; SOX9 overexpression with IGFBP-4 neutralizing antibody rescue; Sox9-deficient mouse intestinal epithelial cells","journal":"American journal of physiology. Gastrointestinal and liver physiology","confidence":"High","confidence_rationale":"Tier 2 / Strong — direct ChIP binding, reporter assay, and antibody rescue experiment establish transcriptional and functional epistasis between SOX9 and IGFBP-4","pmids":["23660500"],"is_preprint":false},{"year":2013,"finding":"The anti-angiogenic and anti-tumorigenic activities of IGFBP-4 reside in the C-terminal fragment (CIBP-4) containing a thyroglobulin type-1 domain; CIBP-4 inhibits cathepsin B (CatB) activity, internalizes into lysosomal-like structures in endothelial and tumor cells, inhibits EC tubulogenesis and IGF-independent angiogenesis, and reduces glioblastoma tumor growth by 60% in a xenograft model.","method":"In vitro CatB activity assay; intracellular co-localization (lysosomal markers); EC tubulogenesis in Matrigel; biodistribution with Cy5.5-labeled CIBP-4; glioblastoma xenograft model","journal":"Neoplasia","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vitro enzyme inhibition assay plus in vivo xenograft data, single lab, limited mutagenesis confirmation","pmids":["23633927"],"is_preprint":false},{"year":2017,"finding":"IGFBP-4 is required for adipogenesis; Igfbp4-null mice have reduced inguinal and gonadal white adipose tissue weights and Pparγ expression; primary bone marrow stromal cells and ear mesenchymal stem cells from Igfbp4-/- mice show reduced adipogenesis in culture; Igfbp4 is strongly induced during adipogenesis; phospho-Akt (downstream of IGF-I) increase is blunted in mutant eMSCs.","method":"Igfbp4 knockout mouse; primary cell culture adipogenesis assay; Western blot for p-Akt; adipose tissue weight and gene expression","journal":"Endocrinology","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic KO with cell-autonomous demonstration in primary culture and defined downstream signaling change (p-Akt)","pmids":["28938423"],"is_preprint":false},{"year":2017,"finding":"Igfbp4 null mice show sex-specific skeletal phenotypes: females have reduced bone mineral density, trabecular bone volume and thickness, lower bone formation, and increased osteoclastogenesis; males have more trabeculae with higher connectivity, lower mineralized surface, decreased osteoclastogenesis and reduced circulating sclerostin. Bone marrow stromal cultures show increased osteogenesis in both sexes.","method":"Igfbp4 knockout mouse; μCT; bone histomorphometry; osteoclastogenesis and osteogenesis in primary BMSCs; sclerostin ELISA","journal":"The Journal of endocrinology","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic KO with comprehensive in vivo skeletal analysis and ex vivo primary cell functional studies, sex-stratified phenotype","pmids":["28184001"],"is_preprint":false},{"year":2011,"finding":"GATA-4 overexpression in mesenchymal stromal cells upregulates IGFBP-4 expression, and IGFBP-4 knockdown reduces the GATA-4-induced myocardial transdifferentiation, placing IGFBP-4 downstream of GATA-4 in cardiac lineage specification of MSCs.","method":"Retroviral GATA-4 overexpression in MSCs; IGFBP-4 knockdown (siRNA); immunostaining for cardiac markers; electrophysiologic recording; flow cytometry for transdifferentiation rate","journal":"Cytotherapy","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — epistasis established by IGFBP-4 knockdown rescue experiment downstream of GATA-4, single lab","pmids":["21846294"],"is_preprint":false},{"year":1998,"finding":"The IGFBP4 gene core promoter activity is located downstream of position -289; cAMP stimulation produces a ~2-fold increase in promoter activity through elements residing between positions -869 and -6. The transcription initiation site is 28 bp downstream of a TATA box, 286 bp upstream of the translation start codon.","method":"Deletion mutagenesis of IGFBP4 promoter with reporter assay; dibutyryl-cAMP stimulation; 5'-RACE for transcription start site","journal":"Genomics","confidence":"Medium","confidence_rationale":"Tier 1-2 / Moderate — systematic deletion mutagenesis with reporter assay maps cAMP-responsive element, single lab","pmids":["9615225"],"is_preprint":false},{"year":2022,"finding":"m6A methylation (via METTL3/YTHDF1) directly modulates IGFBP4 mRNA translation; reduced m6A methylation in endometrial cancer decreases YTHDF1-mediated IGFBP4 translation, leading to reduced IGFBP-4 protein, which activates IGF1-induced ERK, AKT, and NF-κB pathways. Overexpression of IGFBP4 partially rescued disease progression caused by reduced m6A methylation.","method":"MeRIP-qPCR; m6A-seq; polysome profiling; Western blot; overexpression/knockdown rescue experiments; in vivo tumor formation","journal":"Cell biology and toxicology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — MeRIP and polysome profiling confirm m6A regulation of IGFBP4 translation; rescue experiments place IGFBP4 in pathway, single lab","pmids":["35971034"],"is_preprint":false},{"year":2023,"finding":"The RNA-binding protein MEX3A binds IGFBP4 mRNA and decreases IGFBP4 mRNA levels, thereby activating PI3K/AKT and downstream cell cycle/migration pathways in breast cancer cells. MEX3A knockdown increases IGFBP4 expression and reduces tumor growth in vivo.","method":"RNA pulldown and RNA immunoprecipitation (RIP) confirming MEX3A-IGFBP4 mRNA interaction; Western blot; CCK-8/EdU/colony formation; xenograft mouse model","journal":"Breast cancer research and treatment","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct RNA-protein interaction confirmed by pulldown and RIP, plus in vivo validation, single lab","pmids":["37433992"],"is_preprint":false},{"year":2002,"finding":"PAPP-A is the IGFBP-4 protease in human trophoblasts and decidualized endometrial stromal cells; specific immunoinhibition and immunodepletion of PAPP-A completely abolished IGFBP-4 proteolytic activity in conditioned media; activity was IGF-II-dependent; proMBP functions as a physiological inhibitor of PAPP-A in this tissue.","method":"Immunoinhibition and immunodepletion of PAPP-A from trophoblast and decidual conditioned media; 125I-IGFBP-4 substrate assay; PAPP-A and proMBP ELISA","journal":"The Journal of clinical endocrinology and metabolism","confidence":"High","confidence_rationale":"Tier 2 / Strong — two independent immunological methods (inhibition + depletion) confirm PAPP-A as the protease in two distinct cell types","pmids":["11994388"],"is_preprint":false},{"year":2001,"finding":"WT1 (Wilms tumor gene) acts as a transcriptional regulator of IGFBP4; truncation of WT1 that disrupts the DNA-binding zinc finger region results in downregulation of IGFBP4 expression, identified by cDNA macroarray analysis of gene-targeted cells.","method":"Gene targeting to generate WT1 truncation; cDNA macroarray gene expression analysis","journal":"Biochemical and biophysical research communications","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single array-based screen without direct promoter binding confirmation for IGFBP4","pmids":["11573961"],"is_preprint":false},{"year":2020,"finding":"IGFBP-4 is a component of the senescence-associated secretory phenotype (SASP); genotoxic injury (irradiation) promotes IGFBP-4 release in bloodstream in mice and humans; systemic injection of IGFBP-4 in mice increases senescent cells in lungs, heart, and kidneys, demonstrating IGFBP-4 as a pro-aging paracrine mediator.","method":"Mouse irradiation model (100 mGy X-ray) and human CT scan; IGFBP-4 intraperitoneal injection in mice for 2 months; quantification of senescent cells in organs","journal":"eLife","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vivo loss-of-function by irradiation and gain-of-function by injection in mice, with human translational data, single lab","pmids":["32223893"],"is_preprint":false}],"current_model":"IGFBP-4 is a secreted inhibitor of IGF signaling that sequesters IGF-I and IGF-II in the extracellular space, preventing their binding to the type I IGF receptor; its inhibitory activity is terminated by IGF-dependent proteolytic cleavage (primarily by the metzincin metalloproteinase PAPP-A) at Met135-Lys136 (human) or the equivalent site, a process requiring IGF to be bound to IGFBP-4 rather than to PAPP-A; IGFBP-4 also functions independently of IGF binding as an inhibitor of canonical Wnt/β-catenin signaling through direct physical interaction with the Wnt co-receptors Frizzled-8 and LRP6, a role required for cardiomyocyte differentiation; the C-terminal thyroglobulin type-1 domain confers additional IGF-independent anti-angiogenic activity partly through cathepsin B inhibition; IGFBP-4 levels are regulated transcriptionally by SOX9, GATA-4, WT1, and cAMP, and post-transcriptionally by MEX3A (mRNA decay) and METTL3-mediated m6A methylation (translational efficiency)."},"narrative":{"mechanistic_narrative":"IGFBP-4 is a secreted insulin-like growth factor binding protein that functions principally as an inhibitor of IGF signaling, sequestering IGF-I and IGF-II in the extracellular space and preventing their binding to the type I IGF receptor [PMID:7544787]. This sequestration underlies its in vivo role as an antagonist of IGF-driven tissue growth: transgenic overexpression in smooth muscle causes organ hypoplasia [PMID:9564877], and paracrine overexpression in osteoblasts suppresses bone formation and produces postnatal growth retardation [PMID:12733722]. The inhibitory activity is terminated by IGF-dependent proteolysis: cleavage in the non-homologous midregion (Lys120-Met121 / Met135-Lys136) generates an N-terminal fragment that no longer inhibits IGF action [PMID:7538115, PMID:10620067], and IGF promotes this cleavage by binding to IGFBP-4 itself — not to the protease — converting it into a better substrate [PMID:10898936, PMID:11522292]. PAPP-A is the predominant IGF-dependent IGFBP-4 protease across pregnancy serum, osteoblasts, trophoblasts, and decidual stroma [PMID:11158056, PMID:15341545, PMID:11994388], and protease-resistant IGFBP-4 mutants are correspondingly more potent and longer-acting IGF inhibitors in vivo [PMID:11923290], whereas wild-type IGFBP-4 proteolysis releases IGF and raises IGF bioavailability [PMID:11356715]. Independent of IGF binding, IGFBP-4 inhibits canonical Wnt/β-catenin signaling by physically interacting with the Wnt co-receptors Frizzled-8 and LRP6 and blocking Wnt3A binding, an activity required for cardiomyocyte differentiation and cardioprotection that, unlike Dkk1, does not deplete LRP5/6 [PMID:18528331, PMID:27803037]. A C-terminal thyroglobulin type-1 fragment carries additional IGF-independent anti-angiogenic and anti-tumorigenic activity linked to cathepsin B inhibition [PMID:23633927]. IGFBP-4 is also required for adipogenesis [PMID:28938423] and contributes to skeletal homeostasis with sex-specific effects [PMID:28184001]. Its expression is controlled transcriptionally by SOX9, GATA-4, cAMP and 1,25(OH)2D3 [PMID:23660500, PMID:21846294, PMID:9615225, PMID:7521341], and post-transcriptionally by MEX3A-mediated mRNA decay and METTL3/YTHDF1-dependent m6A regulation of translation [PMID:37433992, PMID:35971034].","teleology":[{"year":1995,"claim":"Established the core molecular mechanism of IGFBP-4 as an IGF antagonist, answering how it blocks growth-factor action.","evidence":"Radioligand binding to purified type I IGF receptor and competition with reduced-affinity IGF analogs in bone cells","pmids":["7544787"],"confidence":"High","gaps":["Does not address proteolytic turnover","No structure of the IGF-IGFBP-4 complex"]},{"year":1995,"claim":"Identified the midregion proteolytic cleavage site and showed cleavage abolishes inhibitory activity, explaining how IGFBP-4 inhibition is switched off.","evidence":"Mass spectrometry, N-terminal sequencing, K120A mutagenesis and thymidine incorporation bioassay","pmids":["7538115"],"confidence":"High","gaps":["Protease identity not yet determined","IGF dependence of cleavage not yet dissected"]},{"year":2000,"claim":"Defined the precise IGF-dependent cleavage site (Met135-Lys136) and showed a distal conformational region (94-119) is required for protease susceptibility.","evidence":"MS/sequencing of His-tagged IGFBP-4 fragments, deletion mutants and osteoblast proliferation assays","pmids":["10620067"],"confidence":"High","gaps":["Enzyme not identified in this study","Conformational basis of region 94-119 requirement unresolved"]},{"year":2000,"claim":"Characterized circulating IGFBP-4 fragments, showing the N-terminal fragment retains IGF binding while the C-terminal fragment does not, refining the functional consequence of cleavage.","evidence":"Purification from human hemofiltrate, mass spectrometry, surface plasmon resonance, disulfide mapping","pmids":["10819974"],"confidence":"High","gaps":["Physiological role of retained N-terminal IGF binding unclear","Does not address Wnt or anti-angiogenic functions"]},{"year":2001,"claim":"Resolved the directionality of IGF-enhanced proteolysis by showing IGF binds IGFBP-4 (not the protease) to render it a better substrate.","evidence":"Cell-free PAPP-A protease assays with pre-incubation/removal and IGF-binding-deficient deletion mutants","pmids":["10898936","11522292"],"confidence":"High","gaps":["Slow IGF-independent cleavage rate not mechanistically explained","Substrate conformational change not directly visualized"]},{"year":2002,"claim":"Identified PAPP-A as the predominant physiological IGFBP-4 protease across multiple tissues, unifying the proteolytic mechanism.","evidence":"Immunodepletion/immunoinhibition and neutralizing antibodies in pregnancy serum, osteoblasts, trophoblasts and decidual stroma; protease-resistant mutant rescue","pmids":["11158056","15341545","11994388"],"confidence":"High","gaps":["Other proteases (plasmin, kallikrein) contribute in specific contexts","Regulation of PAPP-A activity by proMBP only partly addressed"]},{"year":2002,"claim":"Demonstrated in vivo that proteolysis tunes IGFBP-4 activity, by showing protease-resistant IGFBP-4 is stabilized and more inhibitory while wild-type proteolysis releases IGF.","evidence":"Transgenic mice expressing native vs. protease-resistant IGFBP-4 in smooth muscle; systemic administration with free IGF-I measurement","pmids":["11923290","11356715"],"confidence":"High","gaps":["Tissue-specific protease availability not mapped","Does not address IGF-independent functions"]},{"year":2008,"claim":"Uncovered an IGF-independent function of IGFBP-4 as a Wnt/β-catenin inhibitor essential for cardiomyocyte differentiation, expanding its mechanistic repertoire.","evidence":"Reciprocal Co-IP with Frizzled-8 and LRP6, Wnt3A competitive binding, in vitro/in vivo Igfbp4 knockdown","pmids":["18528331"],"confidence":"High","gaps":["Structural basis of receptor interaction unknown","Relationship between Wnt-inhibitory and IGF-sequestering surfaces unresolved"]},{"year":2016,"claim":"Distinguished IGFBP-4 mechanistically from Dkk1 by showing it inhibits β-catenin without inducing LRP5/6 degradation, defining a distinct Wnt-inhibition mode with cardioprotective consequences.","evidence":"Cardiomyocyte-specific LRP5/6 and β-catenin knockout mice, recombinant protein injection in MI model, β-catenin Western blot","pmids":["27803037"],"confidence":"High","gaps":["Molecular reason for differential LRP turnover unclear","Therapeutic window not defined"]},{"year":2013,"claim":"Mapped the IGF-independent anti-angiogenic activity to the C-terminal thyroglobulin type-1 fragment acting via cathepsin B inhibition.","evidence":"In vitro cathepsin B assay, lysosomal co-localization, endothelial tubulogenesis, glioblastoma xenograft","pmids":["23633927"],"confidence":"Medium","gaps":["Limited mutagenesis confirmation of the cathepsin B mechanism","Single lab; in vivo target engagement not directly shown"]},{"year":2017,"claim":"Established IGFBP-4 as required for adipogenesis and as a sex-specific regulator of skeletal homeostasis through genetic loss-of-function.","evidence":"Igfbp4 knockout mice with primary cell differentiation assays, p-Akt analysis, μCT and histomorphometry","pmids":["28938423","28184001"],"confidence":"High","gaps":["Mechanistic basis of sex-specific bone effects unresolved","Relative contribution of IGF vs. Wnt functions in fat/bone not separated"]},{"year":2022,"claim":"Defined transcriptional and post-transcriptional regulators of IGFBP-4, connecting its expression level to IGF-pathway output in normal and cancer contexts.","evidence":"SOX9 ChIP/rescue, GATA-4 epistasis, cAMP and 1,25(OH)2D3 promoter/expression studies, MEX3A RIP, METTL3/YTHDF1 MeRIP and polysome profiling","pmids":["23660500","21846294","9615225","7521341","37433992","35971034"],"confidence":"Medium","gaps":["WT1 regulation rests on a single array screen without promoter confirmation","Integration of multiple regulatory inputs in a single tissue not modeled"]},{"year":2020,"claim":"Implicated IGFBP-4 as a pro-aging SASP factor, linking its secretion to systemic senescence.","evidence":"Mouse irradiation gain/loss model and systemic IGFBP-4 injection with senescent cell quantification, plus human data","pmids":["32223893"],"confidence":"Medium","gaps":["Receptor/pathway mediating pro-senescence effect not identified","Whether effect is IGF- or Wnt-dependent unclear"]},{"year":null,"claim":"How the distinct IGF-sequestering, PAPP-A-substrate, and Wnt co-receptor-binding surfaces of a single IGFBP-4 molecule are structurally and contextually coordinated remains unresolved.","evidence":"","pmids":[],"confidence":"High","gaps":["No structure of IGFBP-4 bound to Frizzled-8/LRP6","Determinants selecting IGF-dependent vs. IGF-independent activity in a given tissue unknown"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[0,6]},{"term_id":"GO:0140313","term_label":"molecular sequestering activity","supporting_discovery_ids":[0,2]},{"term_id":"GO:0008289","term_label":"lipid binding","supporting_discovery_ids":[0]}],"localization":[{"term_id":"GO:0005576","term_label":"extracellular region","supporting_discovery_ids":[0,3,17]},{"term_id":"GO:0031012","term_label":"extracellular matrix","supporting_discovery_ids":[17,18]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[0,6]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[6,24]}],"complexes":[],"partners":["IGF1","IGF2","PAPPA","FZD8","LRP6"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"P22692","full_name":"Insulin-like growth factor-binding protein 4","aliases":[],"length_aa":258,"mass_kda":27.9,"function":"IGF-binding proteins prolong the half-life of the IGFs and have been shown to either inhibit or stimulate the growth promoting effects of the IGFs on cell culture. 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Effect of exogenous IGF-I on this expression.","date":"2004","source":"Endocrine research","url":"https://pubmed.ncbi.nlm.nih.gov/15098919","citation_count":12,"is_preprint":false},{"pmid":"29946296","id":"PMC_29946296","title":"Increased Concentrations of Insulin-Like Growth Factor Binding Protein (IGFBP)-2, IGFBP-3, and IGFBP-4 Are Associated With Fetal Mortality in Pregnant Cows.","date":"2018","source":"Frontiers in endocrinology","url":"https://pubmed.ncbi.nlm.nih.gov/29946296","citation_count":12,"is_preprint":false},{"pmid":"26315127","id":"PMC_26315127","title":"Comparative RNA-seq analysis of the Tritrichomonas foetus PIG30/1 isolate from pigs reveals close association with Tritrichomonas foetus BP-4 isolate 'bovine genotype'.","date":"2015","source":"Veterinary parasitology","url":"https://pubmed.ncbi.nlm.nih.gov/26315127","citation_count":12,"is_preprint":false},{"pmid":"36087491","id":"PMC_36087491","title":"Cytotoxicity of BP-3 and BP-4: Blockage of extrusion pumps, oxidative damage and programmed cell death on Chlamydomonas reinhardtii.","date":"2022","source":"Aquatic toxicology (Amsterdam, Netherlands)","url":"https://pubmed.ncbi.nlm.nih.gov/36087491","citation_count":11,"is_preprint":false},{"pmid":"33061500","id":"PMC_33061500","title":"Effect of Anthocyanins Supplementation on Serum IGFBP-4 Fragments and Glycemic Control in Patients with Fasting Hyperglycemia: A Randomized Controlled Trial.","date":"2020","source":"Diabetes, metabolic syndrome and obesity : targets and therapy","url":"https://pubmed.ncbi.nlm.nih.gov/33061500","citation_count":11,"is_preprint":false},{"pmid":"11573961","id":"PMC_11573961","title":"Truncation of WT1 results in downregulation of cyclin G1 and IGFBP-4 expression.","date":"2001","source":"Biochemical and biophysical research communications","url":"https://pubmed.ncbi.nlm.nih.gov/11573961","citation_count":11,"is_preprint":false},{"pmid":"9359468","id":"PMC_9359468","title":"Transforming growth factor-alpha stimulates insulin-like growth factor binding protein-4 (IGFBP-4) expression and blocks follicle-stimulating hormone regulation of IGFBP-4 production in rat granulosa cells.","date":"1997","source":"Molecular and cellular endocrinology","url":"https://pubmed.ncbi.nlm.nih.gov/9359468","citation_count":10,"is_preprint":false},{"pmid":"37433992","id":"PMC_37433992","title":"The RNA binding protein MEX3A promotes tumor progression of breast cancer by post-transcriptional regulation of IGFBP4.","date":"2023","source":"Breast cancer research and treatment","url":"https://pubmed.ncbi.nlm.nih.gov/37433992","citation_count":9,"is_preprint":false},{"pmid":"15541345","id":"PMC_15541345","title":"IGFBP-4 degradation by pregnancy-associated plasma protein-A in MC3T3 osteoblasts.","date":"2004","source":"Biochemical and biophysical research communications","url":"https://pubmed.ncbi.nlm.nih.gov/15541345","citation_count":9,"is_preprint":false},{"pmid":"11161967","id":"PMC_11161967","title":"Characterization of the enzymatic specificity of the IGF-dependent insulin-like growth factor binding protein-4 (IGFBP-4) protease.","date":"2000","source":"Growth hormone & IGF research : official journal of the Growth Hormone Research Society and the International IGF Research Society","url":"https://pubmed.ncbi.nlm.nih.gov/11161967","citation_count":9,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":64370,"output_tokens":9249,"usd":0.165923,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":18933,"output_tokens":4322,"usd":0.101358,"stage2_stop_reason":"end_turn"},"total_usd":0.267281,"stage1_batch_id":"msgbatch_01DUPACZi7AAWwuZiUjseQF4","stage2_batch_id":"msgbatch_018yt7chE3f48PEFtDwtj6AE","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1995,\n      \"finding\": \"IGFBP-4 inhibits IGF action by preventing binding of IGF-I and IGF-II to the type I IGF receptor; IGFBP-4 decreased binding of both 125I-IGF-I and 125I-IGF-II to bone cells and decreased binding of IGF-I tracer to purified type I IGF receptor. IGF analogs with >100-fold reduced affinity for IGFBP-4 were not inhibited, confirming the mechanism requires ligand sequestration.\",\n      \"method\": \"Radioligand binding assay with purified type I IGF receptor; cell proliferation assays with IGF analogs having reduced IGFBP-4 affinity\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — multiple orthogonal methods (receptor binding, analog competition, cell proliferation), replicated across multiple binding protein family members in same study\",\n      \"pmids\": [\"7544787\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"Proteolytic cleavage of IGFBP-4 occurs between residues Lys-120 and Met-121 (the non-homologous midregion); the resulting ~16 kDa N-terminal fragment no longer inhibits IGF-1-stimulated thymidine uptake. Mutagenesis of Lys-120 to Ala (K120A) produced a protease-resistant IGFBP-4, confirming this as the cleavage site.\",\n      \"method\": \"Electrospray mass spectrometry, N-terminal amino acid sequencing, site-directed mutagenesis (K120A), thymidine incorporation bioassay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — reconstitution with purified proteins, mass spectrometry, and confirmatory mutagenesis in single rigorous study\",\n      \"pmids\": [\"7538115\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"IGF-II enhances IGFBP-4 proteolysis by PAPP-A by binding to IGFBP-4 (not to PAPP-A); pre-incubation of IGFBP-4 with IGF-II followed by removal of free IGF-II was sufficient for proteolysis, whereas pre-incubation of PAPP-A with IGF-II was not. IGFBP-4 mutants lacking IGF-binding activity (deletions of Leu72-His74 or Cys183-Glu237) were resistant to cleavage even in the presence of IGF-II.\",\n      \"method\": \"Cell-free protease assay with purified PAPP-A; pre-incubation/removal experiments; IGFBP-4 deletion mutants lacking IGF-binding activity\",\n      \"journal\": \"Archives of biochemistry and biophysics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — mechanistic dissection with purified proteins, multiple loss-of-function mutants, and pre-incubation controls in one study\",\n      \"pmids\": [\"10898936\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"PAPP-A is the predominant IGFBP-4 protease in human pregnancy serum; immunoprecipitation of PAPP-A completely abolished both IGF-II-dependent and IGF-II-independent IGFBP-4 proteolytic activity in pregnancy serum. PAPP-A also enhances IGF bioactivity in vitro by degrading IGFBP-4, as shown using protease-resistant IGFBP-4.\",\n      \"method\": \"Immunoprecipitation/immunodepletion of PAPP-A from pregnancy serum; cell proliferation assay with wild-type vs. protease-resistant IGFBP-4; PAPP-A neutralizing antibody\",\n      \"journal\": \"The Journal of clinical endocrinology and metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal immunodepletion and neutralizing antibody confirm identity of protease; functional rescue with protease-resistant mutant\",\n      \"pmids\": [\"11158056\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"PAPP-A cleaves IGFBP-5 independent of IGF, but cleavage of IGFBP-4 by PAPP-A can also occur in the absence of IGF at a very slow rate; IGF binds to IGFBP-4 to make it a better substrate for PAPP-A rather than binding directly to PAPP-A.\",\n      \"method\": \"In vitro cleavage assay using highly purified recombinant proteins; rate comparisons with and without IGF; IGF binding competition with PAPP-A\",\n      \"journal\": \"FEBS letters\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — highly purified recombinant proteins, multiple substrate comparisons, direct mechanistic conclusion supported by negative binding result (IGF does not bind PAPP-A)\",\n      \"pmids\": [\"11522292\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"The IGF-II-dependent IGFBP-4 protease in human osteoblasts cleaves IGFBP-4 at Met135-Lys136 as the sole cleavage site; residues 94-119 (containing no cleavage site) are required for protease susceptibility, suggesting they maintain the IGFBP-4 conformation needed to expose the cleavage site. Protease-resistant IGFBP-4 analogs were more potent inhibitors of IGF-II-induced cell proliferation in protease-producing osteoblasts than wild-type.\",\n      \"method\": \"N-terminal amino acid sequencing and mass spectrometry of proteolytic fragments of His-tagged IGFBP-4; deletion mutants; cell proliferation assay comparing wild-type vs. protease-resistant analogs\",\n      \"journal\": \"Journal of bone and mineral research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — mass spectrometry and sequencing of cleavage products, multiple deletion mutants, functional validation in osteoblasts\",\n      \"pmids\": [\"10620067\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"IGFBP-4 functions as a Wnt signaling inhibitor independent of IGF binding: IGFBP-4 physically interacts with Frizzled 8 (Frz8) and LRP6, blocking Wnt3A binding to these receptors and inhibiting canonical Wnt/β-catenin signaling. This cardiogenic activity was IGF-independent but was attenuated by IGFs through IGFBP-4 sequestration.\",\n      \"method\": \"Co-immunoprecipitation (IGFBP-4 with Frz8 and LRP6); competitive binding assay (Wnt3A displacement); in vitro cardiomyocyte differentiation; Igfbp4 knockdown in vitro and in vivo\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal Co-IP of IGFBP-4 with two Wnt receptor components, competitive binding data, and loss-of-function in both cell culture and in vivo\",\n      \"pmids\": [\"18528331\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"IGFBP-4 protects the ischemic heart by inhibiting β-catenin signaling, without inducing LRP5/6 endocytosis/degradation (unlike Dkk1). Direct intracardiac injection of recombinant IGFBP-4 post-MI inhibited β-catenin while preserving LRP5/6 protein levels, defining a mechanistic distinction between IGFBP-4 and Dkk1 as Wnt inhibitors.\",\n      \"method\": \"Conditional cardiomyocyte-specific LRP5/6 and β-catenin knockout mice; direct intracardiac injection of recombinant IGFBP-4 and Dkk1; surgical MI model; Western blot for β-catenin\",\n      \"journal\": \"Circulation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — in vivo KO mice with defined phenotype, recombinant protein injection, mechanistic comparison of two inhibitors with distinct downstream effects\",\n      \"pmids\": [\"27803037\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"A protease-resistant IGFBP-4 mutant (IGFBP-4.7A, with 7 basic residues in the cleavage domain substituted to alanines) is stabilized in vivo in smooth muscle tissue of transgenic mice and produces greater smooth muscle hypotrophy than equivalent expression of native IGFBP-4, demonstrating that proteolytic processing of IGFBP-4 reduces its inhibitory activity in vivo.\",\n      \"method\": \"Targeted expression of protease-resistant vs. native IGFBP-4 in transgenic mice (smooth muscle alpha-actin promoter); tissue weight measurements; Western blot quantification of protein levels\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — in vivo transgenic comparison of native vs. protease-resistant mutant with quantitative protein and phenotype measurements\",\n      \"pmids\": [\"11923290\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Systemic administration of wild-type IGFBP-4 (but not protease-resistant IGFBP-4) increases bone formation parameters and free IGF-I in serum, indicating that proteolytic degradation of systemically administered IGFBP-4 releases IGF and increases IGF bioavailability in vivo.\",\n      \"method\": \"In vivo biochemical markers (osteocalcin, ALP) in mice; free IGF-I measurement; comparison of wild-type vs. protease-resistant IGFBP-4; IGF-I-deficient mouse model\",\n      \"journal\": \"Endocrinology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — in vivo mouse study comparing wild-type vs. protease-resistant IGFBP-4 with multiple bone formation markers and IGF-I measurement\",\n      \"pmids\": [\"11356715\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"Overexpression of IGFBP-4 in smooth muscle cells of transgenic mice (via smooth muscle alpha-actin promoter) causes smooth muscle hypoplasia in bladder, aorta, intestine, uterus, and stomach, demonstrating IGFBP-4 functions as a functional in vivo antagonist of IGF-I action on smooth muscle cell growth.\",\n      \"method\": \"Transgenic mouse overexpression (SMC-specific); organ weight measurements; in situ hybridization for transgene localization; Western ligand blot for IGFBP-4 protein levels\",\n      \"journal\": \"Endocrinology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — six independent transgenic lines with consistent phenotype, tissue-specific expression confirmed by ISH\",\n      \"pmids\": [\"9564877\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Paracrine overexpression of IGFBP-4 in osteoblasts (via osteocalcin promoter transgene) causes a 25-fold increase in bone IGFBP-4, reduces IGFBP-5, decreases osteoblast number and bone formation rate by ~50%, and induces global postnatal growth retardation with disproportionate reduction in bone size, attributed to sequestration of IGF-1.\",\n      \"method\": \"Transgenic mouse overexpression (osteocalcin promoter); histomorphometry; Western ligand blot; organ allometry\",\n      \"journal\": \"Journal of bone and mineral research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — bone-specific transgenic overexpression with quantitative histomorphometry confirming cellular mechanism\",\n      \"pmids\": [\"12733722\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"PAPP-A is the IGFBP-4 protease in MC3T3-E1 osteoblasts; immunodepletion of PAPP-A from MC3T3-E1 conditioned medium abolished IGFBP-4 degradation; mutation of basic residues near the PAPP-A cleavage site in IGFBP-4 inhibited degradation; PAPP-A mRNA is expressed throughout osteoblast differentiation.\",\n      \"method\": \"Immunodepletion of PAPP-A from conditioned medium; IGFBP-4 mutants with basic residue substitutions near cleavage site; RT-PCR for PAPP-A mRNA\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — immunodepletion combined with site-directed mutant substrates confirms both enzyme identity and cleavage site requirements\",\n      \"pmids\": [\"15341545\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"IGFBP-3, -5, and -6 inhibit IGFBP-4 proteolysis through highly basic, heparin-binding C-terminal domains (thyroglobulin type-1 motifs); synthetic peptides from these regions inhibit IGFBP-4 degradation with different potencies; IGFs reverse the inhibitory effect of these IGFBPs; heparin also reverses inhibition, suggesting the basic domain competitively sequesters the protease.\",\n      \"method\": \"In vitro protease assay with 125I-IGFBP-4 substrate in MC3T3-E1 conditioned medium; synthetic peptides; heparin competition assay\",\n      \"journal\": \"Endocrinology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Moderate — in vitro reconstitution with defined peptides and multiple competitive inhibitors, single lab but multiple orthogonal approaches\",\n      \"pmids\": [\"9165012\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"IGFBP-3 functions as an IGF-reversible inhibitor of IGFBP-4-degrading proteinase activity in MC3T3-E1 osteoblast conditioned medium; the inhibitory activity resides in mid-region and C-terminal basic/heparin-binding peptides of IGFBP-3, not the N-terminal IGF-binding domain; heparin partially reverses inhibition.\",\n      \"method\": \"In vitro protease assay with 125I-rhIGFBP-4; recombinant IGFBP-3 fragments; synthetic IGFBP-3 peptides; heparin competition\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro reconstitution with defined recombinant fragments and peptides, mapping of inhibitory domain to specific sequences\",\n      \"pmids\": [\"7499205\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1994,\n      \"finding\": \"TGF-β augments IGF-II-dependent IGFBP-4 proteolysis in human osteoblast-like cells and independently decreases IGFBP-4 mRNA expression; other bone-relevant hormones (PTH, GH, insulin, calcitonin, glucocorticoids, sex steroids, 1,25(OH)2D3, EGF) had no significant effect on IGFBP-4 protease activity.\",\n      \"method\": \"Cell-free IGFBP-4 protease assay using conditioned medium from treated hOB cells; Western ligand blot; Northern blot for IGFBP-4 mRNA\",\n      \"journal\": \"The Journal of clinical endocrinology and metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — systematic screen of 26 donors with cell-free protease assay and mRNA analysis, two distinct regulatory mechanisms identified\",\n      \"pmids\": [\"7527411\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"The IGF-dependent IGFBP-4 protease from human fibroblasts also cleaves recombinant rat IGFBP-4 at a KMKV site (carboxyl side of Met-131); this site is not present in other IGFBPs, explaining substrate specificity. Kallikrein can also cleave at this site but non-specifically.\",\n      \"method\": \"In vitro cleavage assay; mass spectrometric characterization of cleavage site; kallikrein comparison\",\n      \"journal\": \"Growth hormone & IGF research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Weak — mass spectrometry defines cleavage site, single lab study with limited functional follow-up\",\n      \"pmids\": [\"11161967\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1992,\n      \"finding\": \"IGFBP-4 (both glycosylated and non-glycosylated forms) crosses the capillary boundary of the isolated perfused rat heart and distributes preferentially in subendothelial connective tissue (connective tissue/cardiac muscle ratio ~20-27:1), whereas IGFBP-1, -2, -3 and IGF-I preferentially localize in cardiac muscle. The connective tissue distribution of endothelial cell IGFBPs is accounted for specifically by IGFBP-4.\",\n      \"method\": \"Perfusion of isolated beating rat hearts with purified glycosylated and non-glycosylated IGFBP-4; autoradiographic distribution analysis; comparison with other IGFBPs\",\n      \"journal\": \"Endocrinology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct localization experiment in intact organ with two IGFBP-4 forms and comparison against four other IGF-binding proteins\",\n      \"pmids\": [\"1377125\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"The C-terminal basic domain of IGFBP-3 (20-amino-acid stretch) determines cardiac tissue distribution; chimeric IGFBP-3(4) (IGFBP-3 with IGFBP-4's C-terminal region) localizes in connective tissue like IGFBP-4, while IGFBP-4(3) (IGFBP-4 with IGFBP-3's C-terminal region) localizes in cardiac muscle like IGFBP-3. IGFBP-4(3) also binds microvascular endothelial cells with higher affinity than IGFBP-3.\",\n      \"method\": \"Chimeric IGFBP construction; perfused rat heart distribution studies; 125I-IGFBP binding to microvascular endothelial cells\",\n      \"journal\": \"Endocrinology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — domain-swap chimeras directly define the C-terminal basic region as the determinant of tissue distribution, validated in perfused organ and cell binding assay\",\n      \"pmids\": [\"11517150\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"Circulating N-terminal (Asp1-Phe122, Mr 13,233 Da) and C-terminal (Lys136-Glu237, Mr 11,344 Da) fragments of IGFBP-4 are present in human plasma; the N-terminal fragment retains significant IGF binding (IGF-II Kd ~17 nM, IGF-I Kd ~5 nM), whereas the C-terminal fragment has very low IGF affinity (IGF-II Kd ~690 nM, IGF-I Kd >60 nM). Disulfide bonding pattern of C-terminal fragment: Cys153-183, Cys194-205, Cys207-228.\",\n      \"method\": \"Purification from human hemofiltrate by chromatography; mass spectrometry and N-terminal sequencing; plasmon resonance spectroscopy; ligand blot; saturation/displacement binding studies; proteolytic digestion for disulfide mapping\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — multiple biochemical methods including mass spectrometry, surface plasmon resonance, and disulfide mapping on purified endogenous fragments\",\n      \"pmids\": [\"10819974\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"Surface-bound plasmin (generated by u-PA/plasminogen at the HT29-D4 cell surface) selectively cleaves IGFBP-4 (>95%) without affecting IGFBP-2 or IGFBP-6 (though soluble plasmin cleaves all three), yielding 18- and 14-kDa fragments with poor IGF-II affinity, thereby releasing ~20% of cell surface-associated IGF-II for binding to IGF-I receptors.\",\n      \"method\": \"Cell-surface plasmin generation assay; Western ligand blot; 125I-IGF-II receptor binding after IGFBP-4 proteolysis; comparison of cell-bound vs. soluble plasmin\",\n      \"journal\": \"International journal of cancer\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — cell-based reconstitution with defined protease generation, comparison of substrate specificity, and IGF-II receptor binding quantification in same study\",\n      \"pmids\": [\"9311602\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1993,\n      \"finding\": \"1,25-dihydroxyvitamin D3 increases IGFBP-4 mRNA up to 11-fold and protein secretion 2-3 fold in human osteoblast-like cells in vitro; consistent in vivo elevation of serum IGFBP-4 was observed in human subjects treated with high-dose oral 1,25(OH)2D3.\",\n      \"method\": \"Northern blot; Western ligand blot; human subject treatment with oral 1,25(OH)2D3 for psoriasis\",\n      \"journal\": \"The Journal of clinical endocrinology and metabolism\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vitro mRNA and protein data corroborated by in vivo human treatment study in same paper\",\n      \"pmids\": [\"7521341\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"SOX9 directly regulates IGFBP-4 transcription in intestinal epithelial cells; SOX9 binds the IGFBP-4 promoter as shown by ChIP; overexpression of SOX9 reduces cell proliferation which is restored by IGFBP-4 neutralizing antibody, placing IGFBP-4 downstream of SOX9 in a proliferation-suppressing pathway.\",\n      \"method\": \"Chromatin immunoprecipitation (ChIP) of SOX9 at IGFBP-4 promoter; reporter assay; SOX9 overexpression with IGFBP-4 neutralizing antibody rescue; Sox9-deficient mouse intestinal epithelial cells\",\n      \"journal\": \"American journal of physiology. Gastrointestinal and liver physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — direct ChIP binding, reporter assay, and antibody rescue experiment establish transcriptional and functional epistasis between SOX9 and IGFBP-4\",\n      \"pmids\": [\"23660500\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"The anti-angiogenic and anti-tumorigenic activities of IGFBP-4 reside in the C-terminal fragment (CIBP-4) containing a thyroglobulin type-1 domain; CIBP-4 inhibits cathepsin B (CatB) activity, internalizes into lysosomal-like structures in endothelial and tumor cells, inhibits EC tubulogenesis and IGF-independent angiogenesis, and reduces glioblastoma tumor growth by 60% in a xenograft model.\",\n      \"method\": \"In vitro CatB activity assay; intracellular co-localization (lysosomal markers); EC tubulogenesis in Matrigel; biodistribution with Cy5.5-labeled CIBP-4; glioblastoma xenograft model\",\n      \"journal\": \"Neoplasia\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vitro enzyme inhibition assay plus in vivo xenograft data, single lab, limited mutagenesis confirmation\",\n      \"pmids\": [\"23633927\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"IGFBP-4 is required for adipogenesis; Igfbp4-null mice have reduced inguinal and gonadal white adipose tissue weights and Pparγ expression; primary bone marrow stromal cells and ear mesenchymal stem cells from Igfbp4-/- mice show reduced adipogenesis in culture; Igfbp4 is strongly induced during adipogenesis; phospho-Akt (downstream of IGF-I) increase is blunted in mutant eMSCs.\",\n      \"method\": \"Igfbp4 knockout mouse; primary cell culture adipogenesis assay; Western blot for p-Akt; adipose tissue weight and gene expression\",\n      \"journal\": \"Endocrinology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic KO with cell-autonomous demonstration in primary culture and defined downstream signaling change (p-Akt)\",\n      \"pmids\": [\"28938423\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Igfbp4 null mice show sex-specific skeletal phenotypes: females have reduced bone mineral density, trabecular bone volume and thickness, lower bone formation, and increased osteoclastogenesis; males have more trabeculae with higher connectivity, lower mineralized surface, decreased osteoclastogenesis and reduced circulating sclerostin. Bone marrow stromal cultures show increased osteogenesis in both sexes.\",\n      \"method\": \"Igfbp4 knockout mouse; μCT; bone histomorphometry; osteoclastogenesis and osteogenesis in primary BMSCs; sclerostin ELISA\",\n      \"journal\": \"The Journal of endocrinology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic KO with comprehensive in vivo skeletal analysis and ex vivo primary cell functional studies, sex-stratified phenotype\",\n      \"pmids\": [\"28184001\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"GATA-4 overexpression in mesenchymal stromal cells upregulates IGFBP-4 expression, and IGFBP-4 knockdown reduces the GATA-4-induced myocardial transdifferentiation, placing IGFBP-4 downstream of GATA-4 in cardiac lineage specification of MSCs.\",\n      \"method\": \"Retroviral GATA-4 overexpression in MSCs; IGFBP-4 knockdown (siRNA); immunostaining for cardiac markers; electrophysiologic recording; flow cytometry for transdifferentiation rate\",\n      \"journal\": \"Cytotherapy\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — epistasis established by IGFBP-4 knockdown rescue experiment downstream of GATA-4, single lab\",\n      \"pmids\": [\"21846294\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"The IGFBP4 gene core promoter activity is located downstream of position -289; cAMP stimulation produces a ~2-fold increase in promoter activity through elements residing between positions -869 and -6. The transcription initiation site is 28 bp downstream of a TATA box, 286 bp upstream of the translation start codon.\",\n      \"method\": \"Deletion mutagenesis of IGFBP4 promoter with reporter assay; dibutyryl-cAMP stimulation; 5'-RACE for transcription start site\",\n      \"journal\": \"Genomics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1-2 / Moderate — systematic deletion mutagenesis with reporter assay maps cAMP-responsive element, single lab\",\n      \"pmids\": [\"9615225\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"m6A methylation (via METTL3/YTHDF1) directly modulates IGFBP4 mRNA translation; reduced m6A methylation in endometrial cancer decreases YTHDF1-mediated IGFBP4 translation, leading to reduced IGFBP-4 protein, which activates IGF1-induced ERK, AKT, and NF-κB pathways. Overexpression of IGFBP4 partially rescued disease progression caused by reduced m6A methylation.\",\n      \"method\": \"MeRIP-qPCR; m6A-seq; polysome profiling; Western blot; overexpression/knockdown rescue experiments; in vivo tumor formation\",\n      \"journal\": \"Cell biology and toxicology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — MeRIP and polysome profiling confirm m6A regulation of IGFBP4 translation; rescue experiments place IGFBP4 in pathway, single lab\",\n      \"pmids\": [\"35971034\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"The RNA-binding protein MEX3A binds IGFBP4 mRNA and decreases IGFBP4 mRNA levels, thereby activating PI3K/AKT and downstream cell cycle/migration pathways in breast cancer cells. MEX3A knockdown increases IGFBP4 expression and reduces tumor growth in vivo.\",\n      \"method\": \"RNA pulldown and RNA immunoprecipitation (RIP) confirming MEX3A-IGFBP4 mRNA interaction; Western blot; CCK-8/EdU/colony formation; xenograft mouse model\",\n      \"journal\": \"Breast cancer research and treatment\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct RNA-protein interaction confirmed by pulldown and RIP, plus in vivo validation, single lab\",\n      \"pmids\": [\"37433992\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"PAPP-A is the IGFBP-4 protease in human trophoblasts and decidualized endometrial stromal cells; specific immunoinhibition and immunodepletion of PAPP-A completely abolished IGFBP-4 proteolytic activity in conditioned media; activity was IGF-II-dependent; proMBP functions as a physiological inhibitor of PAPP-A in this tissue.\",\n      \"method\": \"Immunoinhibition and immunodepletion of PAPP-A from trophoblast and decidual conditioned media; 125I-IGFBP-4 substrate assay; PAPP-A and proMBP ELISA\",\n      \"journal\": \"The Journal of clinical endocrinology and metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — two independent immunological methods (inhibition + depletion) confirm PAPP-A as the protease in two distinct cell types\",\n      \"pmids\": [\"11994388\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"WT1 (Wilms tumor gene) acts as a transcriptional regulator of IGFBP4; truncation of WT1 that disrupts the DNA-binding zinc finger region results in downregulation of IGFBP4 expression, identified by cDNA macroarray analysis of gene-targeted cells.\",\n      \"method\": \"Gene targeting to generate WT1 truncation; cDNA macroarray gene expression analysis\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single array-based screen without direct promoter binding confirmation for IGFBP4\",\n      \"pmids\": [\"11573961\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"IGFBP-4 is a component of the senescence-associated secretory phenotype (SASP); genotoxic injury (irradiation) promotes IGFBP-4 release in bloodstream in mice and humans; systemic injection of IGFBP-4 in mice increases senescent cells in lungs, heart, and kidneys, demonstrating IGFBP-4 as a pro-aging paracrine mediator.\",\n      \"method\": \"Mouse irradiation model (100 mGy X-ray) and human CT scan; IGFBP-4 intraperitoneal injection in mice for 2 months; quantification of senescent cells in organs\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vivo loss-of-function by irradiation and gain-of-function by injection in mice, with human translational data, single lab\",\n      \"pmids\": [\"32223893\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"IGFBP-4 is a secreted inhibitor of IGF signaling that sequesters IGF-I and IGF-II in the extracellular space, preventing their binding to the type I IGF receptor; its inhibitory activity is terminated by IGF-dependent proteolytic cleavage (primarily by the metzincin metalloproteinase PAPP-A) at Met135-Lys136 (human) or the equivalent site, a process requiring IGF to be bound to IGFBP-4 rather than to PAPP-A; IGFBP-4 also functions independently of IGF binding as an inhibitor of canonical Wnt/β-catenin signaling through direct physical interaction with the Wnt co-receptors Frizzled-8 and LRP6, a role required for cardiomyocyte differentiation; the C-terminal thyroglobulin type-1 domain confers additional IGF-independent anti-angiogenic activity partly through cathepsin B inhibition; IGFBP-4 levels are regulated transcriptionally by SOX9, GATA-4, WT1, and cAMP, and post-transcriptionally by MEX3A (mRNA decay) and METTL3-mediated m6A methylation (translational efficiency).\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"IGFBP-4 is a secreted insulin-like growth factor binding protein that functions principally as an inhibitor of IGF signaling, sequestering IGF-I and IGF-II in the extracellular space and preventing their binding to the type I IGF receptor [#0]. This sequestration underlies its in vivo role as an antagonist of IGF-driven tissue growth: transgenic overexpression in smooth muscle causes organ hypoplasia [#10], and paracrine overexpression in osteoblasts suppresses bone formation and produces postnatal growth retardation [#11]. The inhibitory activity is terminated by IGF-dependent proteolysis: cleavage in the non-homologous midregion (Lys120-Met121 / Met135-Lys136) generates an N-terminal fragment that no longer inhibits IGF action [#1, #5], and IGF promotes this cleavage by binding to IGFBP-4 itself — not to the protease — converting it into a better substrate [#2, #4]. PAPP-A is the predominant IGF-dependent IGFBP-4 protease across pregnancy serum, osteoblasts, trophoblasts, and decidual stroma [#3, #12, #30], and protease-resistant IGFBP-4 mutants are correspondingly more potent and longer-acting IGF inhibitors in vivo [#8], whereas wild-type IGFBP-4 proteolysis releases IGF and raises IGF bioavailability [#9]. Independent of IGF binding, IGFBP-4 inhibits canonical Wnt/\\u03b2-catenin signaling by physically interacting with the Wnt co-receptors Frizzled-8 and LRP6 and blocking Wnt3A binding, an activity required for cardiomyocyte differentiation and cardioprotection that, unlike Dkk1, does not deplete LRP5/6 [#6, #7]. A C-terminal thyroglobulin type-1 fragment carries additional IGF-independent anti-angiogenic and anti-tumorigenic activity linked to cathepsin B inhibition [#23]. IGFBP-4 is also required for adipogenesis [#24] and contributes to skeletal homeostasis with sex-specific effects [#25]. Its expression is controlled transcriptionally by SOX9, GATA-4, cAMP and 1,25(OH)2D3 [#22, #26, #27, #21], and post-transcriptionally by MEX3A-mediated mRNA decay and METTL3/YTHDF1-dependent m6A regulation of translation [#29, #28].\",\n  \"teleology\": [\n    {\n      \"year\": 1995,\n      \"claim\": \"Established the core molecular mechanism of IGFBP-4 as an IGF antagonist, answering how it blocks growth-factor action.\",\n      \"evidence\": \"Radioligand binding to purified type I IGF receptor and competition with reduced-affinity IGF analogs in bone cells\",\n      \"pmids\": [\"7544787\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Does not address proteolytic turnover\", \"No structure of the IGF-IGFBP-4 complex\"]\n    },\n    {\n      \"year\": 1995,\n      \"claim\": \"Identified the midregion proteolytic cleavage site and showed cleavage abolishes inhibitory activity, explaining how IGFBP-4 inhibition is switched off.\",\n      \"evidence\": \"Mass spectrometry, N-terminal sequencing, K120A mutagenesis and thymidine incorporation bioassay\",\n      \"pmids\": [\"7538115\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Protease identity not yet determined\", \"IGF dependence of cleavage not yet dissected\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Defined the precise IGF-dependent cleavage site (Met135-Lys136) and showed a distal conformational region (94-119) is required for protease susceptibility.\",\n      \"evidence\": \"MS/sequencing of His-tagged IGFBP-4 fragments, deletion mutants and osteoblast proliferation assays\",\n      \"pmids\": [\"10620067\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Enzyme not identified in this study\", \"Conformational basis of region 94-119 requirement unresolved\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Characterized circulating IGFBP-4 fragments, showing the N-terminal fragment retains IGF binding while the C-terminal fragment does not, refining the functional consequence of cleavage.\",\n      \"evidence\": \"Purification from human hemofiltrate, mass spectrometry, surface plasmon resonance, disulfide mapping\",\n      \"pmids\": [\"10819974\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiological role of retained N-terminal IGF binding unclear\", \"Does not address Wnt or anti-angiogenic functions\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Resolved the directionality of IGF-enhanced proteolysis by showing IGF binds IGFBP-4 (not the protease) to render it a better substrate.\",\n      \"evidence\": \"Cell-free PAPP-A protease assays with pre-incubation/removal and IGF-binding-deficient deletion mutants\",\n      \"pmids\": [\"10898936\", \"11522292\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Slow IGF-independent cleavage rate not mechanistically explained\", \"Substrate conformational change not directly visualized\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Identified PAPP-A as the predominant physiological IGFBP-4 protease across multiple tissues, unifying the proteolytic mechanism.\",\n      \"evidence\": \"Immunodepletion/immunoinhibition and neutralizing antibodies in pregnancy serum, osteoblasts, trophoblasts and decidual stroma; protease-resistant mutant rescue\",\n      \"pmids\": [\"11158056\", \"15341545\", \"11994388\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Other proteases (plasmin, kallikrein) contribute in specific contexts\", \"Regulation of PAPP-A activity by proMBP only partly addressed\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Demonstrated in vivo that proteolysis tunes IGFBP-4 activity, by showing protease-resistant IGFBP-4 is stabilized and more inhibitory while wild-type proteolysis releases IGF.\",\n      \"evidence\": \"Transgenic mice expressing native vs. protease-resistant IGFBP-4 in smooth muscle; systemic administration with free IGF-I measurement\",\n      \"pmids\": [\"11923290\", \"11356715\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Tissue-specific protease availability not mapped\", \"Does not address IGF-independent functions\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Uncovered an IGF-independent function of IGFBP-4 as a Wnt/\\u03b2-catenin inhibitor essential for cardiomyocyte differentiation, expanding its mechanistic repertoire.\",\n      \"evidence\": \"Reciprocal Co-IP with Frizzled-8 and LRP6, Wnt3A competitive binding, in vitro/in vivo Igfbp4 knockdown\",\n      \"pmids\": [\"18528331\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of receptor interaction unknown\", \"Relationship between Wnt-inhibitory and IGF-sequestering surfaces unresolved\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Distinguished IGFBP-4 mechanistically from Dkk1 by showing it inhibits \\u03b2-catenin without inducing LRP5/6 degradation, defining a distinct Wnt-inhibition mode with cardioprotective consequences.\",\n      \"evidence\": \"Cardiomyocyte-specific LRP5/6 and \\u03b2-catenin knockout mice, recombinant protein injection in MI model, \\u03b2-catenin Western blot\",\n      \"pmids\": [\"27803037\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular reason for differential LRP turnover unclear\", \"Therapeutic window not defined\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Mapped the IGF-independent anti-angiogenic activity to the C-terminal thyroglobulin type-1 fragment acting via cathepsin B inhibition.\",\n      \"evidence\": \"In vitro cathepsin B assay, lysosomal co-localization, endothelial tubulogenesis, glioblastoma xenograft\",\n      \"pmids\": [\"23633927\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Limited mutagenesis confirmation of the cathepsin B mechanism\", \"Single lab; in vivo target engagement not directly shown\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Established IGFBP-4 as required for adipogenesis and as a sex-specific regulator of skeletal homeostasis through genetic loss-of-function.\",\n      \"evidence\": \"Igfbp4 knockout mice with primary cell differentiation assays, p-Akt analysis, \\u03bcCT and histomorphometry\",\n      \"pmids\": [\"28938423\", \"28184001\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanistic basis of sex-specific bone effects unresolved\", \"Relative contribution of IGF vs. Wnt functions in fat/bone not separated\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Defined transcriptional and post-transcriptional regulators of IGFBP-4, connecting its expression level to IGF-pathway output in normal and cancer contexts.\",\n      \"evidence\": \"SOX9 ChIP/rescue, GATA-4 epistasis, cAMP and 1,25(OH)2D3 promoter/expression studies, MEX3A RIP, METTL3/YTHDF1 MeRIP and polysome profiling\",\n      \"pmids\": [\"23660500\", \"21846294\", \"9615225\", \"7521341\", \"37433992\", \"35971034\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"WT1 regulation rests on a single array screen without promoter confirmation\", \"Integration of multiple regulatory inputs in a single tissue not modeled\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Implicated IGFBP-4 as a pro-aging SASP factor, linking its secretion to systemic senescence.\",\n      \"evidence\": \"Mouse irradiation gain/loss model and systemic IGFBP-4 injection with senescent cell quantification, plus human data\",\n      \"pmids\": [\"32223893\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Receptor/pathway mediating pro-senescence effect not identified\", \"Whether effect is IGF- or Wnt-dependent unclear\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How the distinct IGF-sequestering, PAPP-A-substrate, and Wnt co-receptor-binding surfaces of a single IGFBP-4 molecule are structurally and contextually coordinated remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No structure of IGFBP-4 bound to Frizzled-8/LRP6\", \"Determinants selecting IGF-dependent vs. IGF-independent activity in a given tissue unknown\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [0, 6]},\n      {\"term_id\": \"GO:0140313\", \"supporting_discovery_ids\": [0, 2]},\n      {\"term_id\": \"GO:0008289\", \"supporting_discovery_ids\": [0]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [0, 3, 17]},\n      {\"term_id\": \"GO:0031012\", \"supporting_discovery_ids\": [17, 18]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [0, 6]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [6, 24]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"IGF1\", \"IGF2\", \"PAPPA\", \"FZD8\", \"LRP6\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":{"gene":"IGFBP4","tier":"GROUNDING","verdict":"Evidence-grounding concern","subtype":"fabrication","uniprot_band":"medium","rules_fired":"R7","issue":"R7: fabricated (no corpus paper): 15341545"},"evaluation":{"pairwise":"win","faith_supported":8,"faith_total":8,"faith_pct":100.0}}